Size

I originally imagined the Shambles as mind-bogglingly huge creatures of 30 miles in diameter. I think that's a neat idea, but when I force myself to try and imagine it. Well. My mind boggles. Which disrupts the imagination somewhat.

So I think I'm going to tone that down. For now, I'm going to work on the premise of Shambles being of varying sizes but all around 1-3 miles in diameter. This is a tenfold, or more, size reduction and will have various effects in the world.

According to Medieval Demographics Made Easy, a "big" city by medieval standards is 12,000 plus. And a typical city of 10,000 people would have filled roughly a quarter mile of land. A thriving metropolis would have 40,000 (the max size of medieval London) people and fill a square mile, which fits nicely on the back of a 1 mile diameter golem.

Green Spaces

One major effect is that the Shambles will have significantly fewer green areas. There will be spots of land set aside for limited groves and water sources, but not much else. This means the people on the backs of the Shambles depend heavily upon trade with the people below in the Wastes. The people in the Wastes depend on the tradesmen and craftsmen on the Shambles for their tools and equipment.

Population

The population on the backs of the Shambles is going to vary depending on food sources, population density, and size of the Shamble. Before, they were going to be sprawling masses with numerous villages and towns on the backs. Now, the Shambles will have one major city, and nothing else.
The rest of the population of the world lives either high in the mountains, in the dark elf empire, or in the Wastes.

Food and Trade

People in the Wastes who try and build permanent structures, find those structures trampled within a year or two. People have stopped trying. The best they can do is have semi-permanent structures they can move with a day or two of notice. The pace of the Shambles makes them an implacable threat but not a surprising one.

The people of the Wastes have enough time to move any structures and livestock before the Shambles reach them. And then they can trade with the people of the Shambles. Those atop the Shambles need food, and those who live in the Wastes find value in the craftsmanship afforded by city life and permanent structures. Once the Shamble moves on, the people below know they have at least a year before the Shamble returns, at least most of the time. This is enough time for them to have a growing season or two and for them to tend to herds. It is not an easy life, but it is a sustainable one.

Some druids also, for various reasons, roam the Wastes trying to maintain or restore the ecosystem. Parts of Vernlarum are completely desert, but much of the known world is a grassy plain.